The relatively low frequency of moderate-to-strong earthquakes in the eastern US compared with shaking and volcanism along the the West Coast is due to significant differences in the the geological activity taking place beneath their residents' feet.

In California's case, the western edge of the state straddles an active boundary between two enormous plates in Earth's crust – the North American plate and the Pacific Plate. The San Andreas fault and its tributaries mark the boundary with plenty of shaking and mountain-building as the Pacific Plate grinds against its way north against the continent.

Why quakes happen on the East Coast

The eastern seaboard, by contrast, sits in the middle of the North American Plate, but 200 million to 300 million years ago it was along an ancient plate boundary that ran through a connected Europe, Africa, and North America.

Both the collisions and the splitting generated a network of faults under what is now the East Coast.

Over the intervening millenniums, the Appalachians eroded, providing the soil along the Piedmont that, at Charleston, is some 3,000 feet deep. Meanwhile, the crust cooled and became more rigid, locking the faults in place, but remaining generally inactive.

These faults, however, are still susceptible to stresses on the crust. Some of these stresses can come from the mid-Atlantic Ridge, a structure that looks like a zipper and runs north and south along the Atlantic Ocean floor. It is constantly oozing molten material from deep in the earth, forming fresh crust and pushing North America and Europe farther apart at the rate of about 17 millimeters a year.

When that stress builds along a fault zone with the right orientation, "occasionally it might rupture," adds Chuck Bailey, a geology professor at the College of William and Mary in Williamsburg, Va. But, he adds, the fault zones most susceptible to occasional quakes are not uniformly spread throughout the eastern US. And they don't always manifest themselves in easy to spot features on the surface, the way faults do in the West.

Why it was felt so far away

Some of the quake activity, especially in the Northeast and into Canada, also can be traced to the melting of continental glaciers after the last Ice Age. Hudson Bay is rising about 2 centimeters a year – the crust rising in a process akin to releasing a squeezed ball. That also adds to the stresses on the crust.

Tuesday's quake was felt across a wide swath of the region because the rock formations deep beneath the surface are old and cold, and so transmit seismic waves more efficiently than thick layers of ancient sediment.

For easterners not used to thinking about earthquakes, Tuesday's shaking should serve as a reminder than quakes are more than theoretical occurrences, Dr. Bailey says.

The event also "may produce a focus of energy" on the part of researchers "to think about why we do get earthquakes in these regions that are far away from plate boundaries."